1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus for polishing an object to be polished (substrate) such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multilayer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). Thus, there has been employed a chemical mechanical polishing apparatus for planarizing a surface of a semiconductor wafer. In the chemical mechanical polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding device, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing surface under a predetermined pressure by the substrate holding device. At this time, the polishing table and the substrate holding device are moved relative to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
In such polishing apparatus, if the relative pressing force applied between the semiconductor wafer, being polished, and the polishing surface of the polishing pad is not uniform over the entire surface of the semiconductor wafer, then the surface of the semiconductor wafer is polished insufficiently or excessively in different regions thereof depending on the pressing force applied thereto. It has been customary to uniformize the pressing force applied to the semiconductor wafer by providing a pressure chamber formed by an elastic membrane at the lower portion of the substrate holding device and supplying the pressure chamber with a fluid such as air to press the semiconductor wafer under a fluid pressure through the elastic membrane, as seen in Japanese laid-open patent publication No. 2006-255851.
In this case, the polishing pad is so elastic that pressing forces applied to a peripheral portion of the semiconductor wafer being polished become non-uniform, and hence only the peripheral portion of the semiconductor wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, the retainer ring for holding the peripheral edge of the semiconductor wafer is vertically movable with respect to the top ring body (or carrier head body) to press an annular portion of the polishing surface of the polishing pad that corresponds to the peripheral portion of the semiconductor wafer by the retainer ring.
In the conventional polishing apparatus, a lateral force (horizontal force) is applied to the retainer ring by a frictional force between the semiconductor wafer and the polishing surface of the polishing pad during polishing, and the lateral force (horizontal force) is received by a retainer ring guide provided at an outer circumferential side of the retainer ring. Therefore, when the retainer ring is vertically moved to follow undulation of the polishing surface of the polishing pad, a large frictional force is generated at sliding contact surfaces of an outer circumferential surface of the retainer ring and an inner circumferential surface of the retainer ring guide. Thus, the following capability of the retainer ring becomes insufficient, and a desired surface pressure of the retainer ring cannot be applied to the polishing surface of the polishing pad.
Further, in order to transmit a rotative force from the top ring (or carrier head) to the retainer ring, a rotary drive unit such as driving pins is provided between the retainer ring and the retainer ring guide. When the retainer ring is vertically moved, a large frictional force is generated at the rotary drive unit. Thus, the following capability of the retainer ring becomes insufficient, and a desired surface pressure of the retainer ring cannot be applied to the polishing surface of the polishing pad.